Planet gear and use thereof

ABSTRACT

A planetary gear comprising a central, rotatable input/output shaft ( 1   a ), an elastic outer ring ( 4 ) which when being elastically deformed encloses and presses at least three planet wheels ( 6, 12 ) in the form of rollers radially against a centrally mounted sun shaft ( 2 ), which extends axially in alignment with the input/output shaft ( 1   a ). The planet wheels ( 6, 12 ) have central holes and are rotatably mounted on stays ( 7, 13 ), the centre lines of the stays ( 7, 13 ) being parallel with the sun shaft ( 2 ). At least two ( 12 ) of the planet wheels have substantially coinciding centres with the stays ( 13 ) associated with the planet wheels ( 12 ), while the planet wheel or other planet wheels ( 6 ) as well as the associated stay or stays ( 7 ) is/are constructed such that the centre lines of the stays are positioned a given distance closer to the sun wheel ( 2 ) than the axes of the planet wheels ( 6 ) when the planet wheels ( 6 ) engage the sun shaft ( 2 ). This results in an optimum transfer of moment, while achieving an accurate positioning of the sun shaft as well as the planet wheel or the other planet wheels against the at least two planet wheels having centres which coincide with the respective stays.

THE PRIOR ART

[0001] The invention relates to a planetary gear having a central,rotatably mounted sun wheel, at least three planet wheels arrangedaround the sun wheel which are rotatable about their own axes, means forresiliently pressing the peripheral outer faces, which are in the formof faces of revolution, of the planet wheels into force transferringengagement with the peripheral outer face of the sun wheel.

[0002] A planetary gear of the type stated above is known from U.S. Pat.No. 2,344,078 which is used for driving a centrifugal charger for aninternal combustion engine. In this planetary gear, the stays of theplant wheels are mounted on a carrier which is firmly connected with theinput shaft, and the centre lines of the stays are located on a pitchdiameter which coincides with the axes of the planet wheels when theplanet wheels engage the sun shaft. The length of the sun shaft, whichis in contact with the planet wheels, is adapted such that the frictionbetween them ensures the transfer of moment. By allowing the planetwheels to rotate about fixed stays a constant force is achieved betweenplanet wheels and sun shaft.

[0003] In a centrifugal charger, increasing transfer of moment isrequired with in creasing speed. In the above-mentioned known planetarygear, however, decreasing transfer of moment is achieved at anincreasing speed because of the centrifugal forces, since, at highspeeds, the centrifugal forces on the planet wheels reduce the availablefriction between sun shaft and planet wheels like the planetary geardiscussed above.

[0004] Moreover, a planetary gear of the type stated initially is knownfrom DK 171,047 B1. In this known planetary gear it is intended toautomatically adapt the transferred moment to the need even at highspeeds, which is achieved in this prior art in that the diameters of thestays are smaller than the holes of the planet wheels, and in that thecentre lines of the stays are located closer to the sun shaft than theaxes of the planet wheels when the planet wheels engage the sun shaft.

OBJECT OF THE INVENTION

[0005] It is an object of the invention to provide a planetary gearwhich ensures an optimum transfer of moment relative to the art thusknown, while ensuring an exact positioning of the planet wheels in theplanetary gear in all operational conditions, thereby preventingoccurrence of operational problems because of the mounting of the planetwheels and the sun shaft taught by the prior art.

[0006] This object is achieved according to the invention in that atleast one and maximum two of the planet wheels is/are mounted such thatits axis/their axes of rotation is/are substantially fixed relatively tothe axis of rotation of the sun wheel, while each of the other planetwheels is arranged on a shaft part extending with a predetermined playinto a central bore provided in the planet wheel concerned, such thatthe radial distance between the centre line of the shaft part and theaxis of rotation of the sun wheel is smaller by a predetermined valuethan the corresponding radial distance between the centre line of theplanet wheel concerned and the axis of rotation of the sun wheel whenthe planetary gear is in its operational state, in which all the planetwheels are in engagement with the sun wheel, and each of the shaft partsis in engagement with the inner surface of the corresponding bore.

[0007] This ensures that the planet wheel or wheels mounted by means ofa stay or stays whose centre lines are located a given distance closerto the sun shaft than the axes of the planet wheels when the planetwheels engage the sun shaft, will give a “ramp effect”, whereby thetransferred moment is adapted to the need, while this planet wheel orthese planet wheels will press the sun shaft inwards against the twoplanet wheels which are positioned with centres coinciding with theirrespective stays. This will ensure an exact positioning of both sunshaft and the other planet wheels against thee two planet wheels, which,as mentioned, are positioned by means of their respective stays.

[0008] When, as stated in claim 2, the given distance is in excess ofthe normal accuracy of the machining, preferably in excess of about 0.01mm, the given difference in distance is greater than the normal accuracyof the machining and preferably greater than 0.01 mm.

[0009] When, as stated in claim 3, the given distance is in a range ofsubstantially 0.1-2.0 per thousand of the radial dimension of theplanetary gear and preferably in a range of substantially 0.5-1.0 perthousand, an expedient moment transfer function will be achieved, sincethe moment is adjusted to the given need.

[0010] When, as stated in claim 4, the planet wheel or the other planetwheels as well as the associated stay or stays is/are constructed suchthat the stay or stays is/are formed with an outer diameter which issmaller than the inner diameter of the corresponding wheel in the planetwheel or wheels, the difference in diameters being in excess of thenormal machining accuracy, preferably in excess of about 0.01 mm, thedesired relation between the centre line/lines of the stay/stays and theaxis/axes of the planet wheel/wheels may be achieved.

[0011] When, as stated in claim 5, the planet wheel or the other planetwheels as well as the associated stay or stays is/are constructed suchthat each stay comprises an eccentric mounted rotatably thereon, thedesired ramp effect is achieved. Simultaneously, the planet wheel or theother planet wheels may be constructed identically with thefirst-mentioned ones, as the outer diameter of the eccentric body maycorrespond to the outer diameter of a stay for the first-mentionedplanet wheels, thereby facilitating the production.

[0012] When, as stated in claim 6, the centre of the stay is positionedcloser to the centre of the sun shaft than the centre of the associatedeccentric, an expedient structure is achieved.

[0013] When, as stated in claim 7, the centre of the stay is positionedfurther away from the centre of the sun shaft than the centre of theassociated eccentric, it is additionally ensured that an overloadprotection may be achieved.

[0014] When, as stated in claim 8, the planet wheels are constructedsubstantially identically, and they are preferably constructed asbearings, preferably roller bearings whose inner diameter substantiallycorresponds to the outer diameter of the stays, and whose outer bearingring serves as a planet wheel, a saving and a simpler production of theplanetary gear are achieved, since different planet wheels need not beproduced.

[0015] Finally, the invention relates to a use of the planetary gear asdefined in claim 9 where the planetary gear may be used for transferringmoment at high gear ratios, e.g. up to 13:1, and/or at a relativelygreat transfer of moment using a relatively small volume, such as fortransferring moment to compressors for internal combustion engines orfor transferring moment to propulsion wheels in vehicles.

THE DRAWING

[0016] Embodiments of the invention will be described more fully belowwith reference to the drawing, in which

[0017]FIG. 1 shows a sectional view of a planetary gear according to theinvention seen in the direction (I-I in FIG. 3,

[0018]FIG. 2 shows the forces between the outer ring, a planet wheel andthe sun shaft in the planetary gear according to the invention,

[0019]FIG. 3 shows a sectional view of the outer ring and the planetwheels in the planetary gear seen in the direction 111-111 in FIG. 1,

[0020]FIGS. 4 and 5 show sections corresponding to FIG. 1, but ofanother embodiment of the invention, seen in the direction IV-IV in FIG.6 and V-V in FIG. 7, respectively,

[0021]FIGS. 6 and 7 show sectional views corresponding to FIG. 3, but ofan other embodiment of the invention, seen in the direction VI-VI inFIG. 4 and VII-VII in FIG. 5, respectively,

[0022]FIGS. 8a-c are sketch views of a section of an outer ring, a sunshaft and a planet wheel in a planetary gear according to the invention,illustrating the distance conditions for achieving the ramp effect, and

[0023]FIG. 9 shows a curve of the theoretically transferred force in aplanetary gear according to the invention.

DESCRIPTION OF THE EMBODIMENTS

[0024]FIG. 1 shows a section through a planetary gear according to anembodiment of the invention. The gear comprises a circular disc-shapedcarrier 1 having on one side a protruding stub shaft 1 a which forms aninput shaft whose rotation is to be converted into rotation of a sunshaft 2, whose one end extends out of the gear, said sun shaft 2 beingaxially aligned with the input stub shaft 1 a.

[0025] Equidistantly spaced near the circumference of the carrier 1 is aplurality of holes 3, whose number is e.g. twelve as shown in FIG. 3.Also provided is an elastic outer ring 4 which has pins 5 on the sidefacing the carrier 1, said pins corresponding in number to the holes 3and extending into these.

[0026] The pins 5 have a smaller diameter than the holes 3 so that theelastic outer ring 4 may be moved and deformed slightly relative to thecarrier 1. The engagement of the pins 5 with the holes 3 causes theouter ring 4 to be driven by the carrier 1 when this is rotated.

[0027] The outer ring 4 encloses a planet wheel 6 as well as two planetwheels 12, as is shown in FIG. 3. The planet wheels are constructed asrigid bushings or rollers which engage the sun shaft 2 arrangedcentrally between the planet wheels 6 and 12.

[0028] The planet wheels 6 and 12 have central holes and are mounted onstays 7 and 13, respectively. The planet wheel 6 is mounted loosely onthe stay 7 whose outer diameter is slightly smaller than the hole 6′ inthe planet wheel 6, while the planet wheels 12 are mounted rotatably andwithout play on the stays 13. The centre line of stay 7 of the planetwheel 6 is therefore parallel with, but does not coincide with the axisof the planet wheel 6, while the centre lines of the stays 13 of theplanet wheels 12 coincide with or substantially coincide with the axesof the planet wheels 12.

[0029] The stays 7 and 13 are controlled in a frame 8 which consists ofa first frame part 8 a and a second frame part 8 b, whereby also theplanet wheels 6 and 12 are controlled within the elastic outer ring 4.

[0030] The two frame parts 8 a, 8 b are assembled by means of controlbolts 9 whose cylindrical stems fit snugly in bores in the frame partsto ensure their mutually exact position.

[0031] To ensure the axial position of the sun shaft 2 in the gear, thesun shaft has a collar 2′ on the end extending into the gear. Thiscollar engages one side face of a slide disc 10, whose opposite secondside face engages the axial, internal ends of the planet wheels 6 and12.

[0032] Further, in particular in operation, the sun shaft 2 will bepositioned by the planet wheel 6 which will press the sun shaft 2inwards against the two planet wheels 12 which are mounted without playor substantially without play on the stays 13. It is observed that thereare no other forms of bearings for controlling the sun shaft 2.

[0033] Prior to mounting, the inner diameter of the outer ring 4 isslightly smaller than the diameter of the circle that is tangent to thethree planet wheels 6 and 12. It will be appreciated that the outer ring4 when being elastically deformed presses the planet wheels 6 and 12against the centrally arranged sun shaft 2.

[0034] Radial forces are created hereby between the outer ring 4, theplanet wheels 6 and 12 as well as the sun shaft 2. These normal forces,together with the traction oil filled in the gearbox, ensure frictionbetween the components of the gear, thereby causing the sun shaft 2 torotate when the outer ring 4 is rotated by the carrier 1.

[0035]FIG. 2 shows the forces between an outer ring 4, a planet wheel 6and a sun shaft 2. The situation where the gear does not rotate andeverything is in balance, is shown in solid line. In operation, theplanet wheel 6 will be displaced to a position which is shown in dashedline. It will be seen clearly that the planet wheel 6 is pressed againstthe sun shaft 2 by the bias of the elastic outer ring 4, and that thehole 6′ in the planet wheel 6 is larger than the diameter of the stay 7,even though this difference is somewhat exaggerated in FIG. 2 forclarity.

[0036] The bias of the elastic outer ring 4 results in the generation ofcomponents which are designated as follows in FIG. 2:

[0037] Fnr=the normal force on the planet wheel from the outer ring,

[0038] Fns=the normal force on the planet wheel from the sun shaft,

[0039] Ftr=the tangential force between the planet wheel and the outerring, and

[0040] Fts=the tangential force between the planet wheel and the sunshaft.

[0041] Since the equilibrium of forces must be fulfilled for a planetwheel, the resulting component in the direction (Fn) of the normalforces must be equal to the sum of the normal forces (Fnr, Fns), and theresulting component in the direction (Ft) of the tangential forces mustbe equal to the sum of the tangential forces (Ftr, Fts).

[0042] No centripetal forces occur, because the planet wheels 6 areloosely mounted on fixed stays 7.

[0043] The following linkage exists between the two components of force$\frac{{Fns} - {Fnr}}{{Ftr} + {Fts}} = {{tangent}\quad v}$

[0044] or since Fts≅Ftr

Fns≅tangent v·2Ftr+Fnr

[0045] It will be seen by considering the components of force in FIG. 2that the radial position of the stay 7 relative to the planet wheel 6contributes to determining the transferable moment, as the frictionalforce is proportional to the normal force.

[0046] If the centre line of the stay 7 is moved outwards so as tocoincide with the centre line of the planet wheel 6, then v=0, henceFns=Fnr, and there is thus no ramp effect.

[0047] Where it is described above that the planet wheel 6 is mountedloosely on the associated stay 7, as the outer diameter of the stay isslightly smaller than the inner diameter of the hole in the planetwheel, it will of course be a matter of differences that are greaterthan the tolerances which exist in the structure. This means that thedifferences in diameters are in excess of the ordinary machiningaccuracy. Preferably, a difference may be involved which is greater thanabout 0.01 mm at the sizes involved here, which means e.g. a planetarygear having an outer diameter of the order of about 100 mm.

[0048] The same will apply to the distance by which the centre of one ormore of the stays is displaced inwards toward the sun shaft, since heretoo a distance must be involved which is in excess of the normalmachining accuracy. Preferably the distance is in excess of about 0.01mm, e.g. in case of a planetary gear having an outer diameter of theorder of about 100 mm. The distance may also be stated as being in arange of substantially 0.1-2.0 per thousand of the radial dimension ofthe gear and preferably in a range of substantially 0.5-1.0 perthousand.

[0049]FIG. 3 is an image of the carrier 1 and the elastic outer ring 4in a position of rest.

[0050] It will be seen that in position (A) during rotation the holes 3of the carrier 1 will press radially inwards against the pins 5 of theouter ring 4, and that in position (B) during rotation the holes 3 ofthe carrier 1 will exert a radially outward pull in the pins 5 of theouter ring 4.

[0051] The mentioned second frame part 8 b may serve as a suspension forthe gear, when this frame part 8 b is secured in any suitable manner toa frame not shown in the drawings.

[0052] The diameter of the sun shaft 2 and the diameter of the planetwheels 6 are adjusted so as to achieve the desired gear ratio of therevolutions of the input stub shaft 1 a and the sun wheel 2.

[0053] The part of the surface of the sun shaft 2 which is in contactwith the planet wheels 6, may optionally be provided with a coatingwhich can increase the friction.

[0054] The radial inner side of the outer ring 4 is advantageouslyprovided with annular grooves 11 which prevent oil planing at highspeeds, allowing the use of several different types of oil.

[0055] In a further embodiment, which is not shown in the drawing, thecarrier is constructed to enclose the radial outer side of the outerring, the carrier being provided with a collar which extends along theouter ring. The radial inner side of the collar carries elements in theform of pins or rollers whose engagement with the outer ring ispositioned on a pitch circle which is smaller than the outer diameter ofthe outer ring in the unloaded state of the outer ring. This providesstabilization of the outer ring.

[0056] An alternative embodiment of the invention is shown in FIGS. 4-7,where parts constructed in a manner similar to FIGS. 1 and 3 aredesignated by the same reference numerals. Thus, this embodiment, too,involves a disc-shaped carrier 1 having a protruding stub shaft 1 a aswell as a protruding end of the sun shaft 2 on the other side. Thisembodiment likewise has an elastic outer ring 4 with pins 5 extendinginto holes 3 in the carrier 1. Also, there is a plurality of planetwheels 6 and 12 positioned inside the gear, and a frame 8 comprising twoparts 8 a and 8 b controls the planet wheels by means of stays 7 and 13.The outer ring 4 may moreover be provided with annular grooves 11 on itsinner face to prevent oil planing at high speeds.

[0057] Instead of the above-mentioned stay 7 whose outer diameter issmaller than the hole 6′ in the planet wheel 6, this embodiment uses astay 7′ having an eccentric body 15. The stay 7′ is carried by the frame8, and the eccentric body 15 is mounted rotatably on the stay, but fixedrelative to the inner part of the planet wheel.

[0058] Further, a ball or roller bearing, whose outer ring serves as aplanet wheel 6, is shrunk on the eccentric 15.

[0059] As will appear from FIGS. 4 and 6, a ramp effect may be achievedalso in this manner, since the distance from the centre of the stay 7′to the centre of the sun shaft 2 is smaller than the distance from thecentre of the planet wheel 6 to the centre of the sun shaft 2. This isensured in that the centre of the axis of the stay 7′ is positionedcloser to the sun shaft than the centre of the eccentric 15. This centrewill be able to move and thereby ensure the ramp effect when the planetwheel 6 engages the sun shaft 2.

[0060] In this embodiment, the planet wheels 12 and 6 may be constructedidentically. The planet wheels 12 may e.g. be constructed in the samemanner as the planet wheel 6, whose outer bearing ring serves as aplanet wheel, and be formed with inwardly pressed roller bearings 16whose bearing ring is shrunk on the stays 13.

[0061] Finally, a planetary gear according to the invention may beconstructed as shown in FIGS. 5 and 7, which correspond to theembodiment shown in FIG. 4, except that here the centre of the stay 7′is positioned further away from the sun shaft 2 than the centre of theeccentric body 15, but still so that the distance from the centre lineof the planet wheel 6 to the sun shaft 2 is smaller than the distancesfrom the centre lines of the planet wheels 12 to the sun shaft 2. This,too, will result in a ramp effect, but simultaneously an overloadprotection will be achieved. By suitable dimensioning of the distancesand dimensions, the ramp effect will just amount to a certain limitvalue where the centre of the eccentric will move to the other side ofthe line through the centre of the sun shaft 2 and of the stay 7′. Then,the possibility of moment transfer will be removed.

[0062]FIGS. 8a-8 c are sketch views of a section of an outer ring 4, asun shaft 2 and planet wheel 6 in a planetary gear according to theinvention, illustrating the distance conditions for achieving the rampeffect. The example shown illustrates an embodiment having a stay 7 witha smaller external diameter than the hole 6′ in the planet wheel 6, butthe same conditions will apply to this embodiment where an eccentric 15is used. As will be seen, the radius of the circle with the centre inthe sun shaft 2 and the circumference through the centre of the planetwheel 6 is called PCDR, and the radius of the circle with the samecentre and circumference through the centre of the stay is called PCDS.The difference between these two is called a, and the distance betweenthe centre of the planet wheel 6 and the centre of the stay 7 is calledr. If, as shown in FIG. 8c, the stay 7 engages the inner side of theplanet wheel 6 at the sun shaft 2, which means in the position of rest,then PCD_(s)=PCD_(s min), and a will be equal to r.

[0063]FIG. 8a shows the conditions when a=0, while FIG. 8b shows theconditions when a is greater than 0, but smaller than r. Finally, FIG.8c shows the conditions when a=r. In the invention, as will be seen, itis so that the distance r will be greater than the distance a is inoperation.

[0064] The force that can be transferred in a planetary gear accordingto the invention may be calculated by means of the mentioned quantities,the result being dependent on the difference between r and a. An exampleof such a calculation is shown in FIG. 9, in which theoretical forcetransferred to a centre shaft as a function of a is calculated for aplanetary gear having an outer ring with an inner diameter D of 90 mm,where r=0.09 mm, and where the moment from the outer ring on the planetwheel is 35 Nm. As will be seen, the force transferred will be minimumand exclusively depend on the bias from the outer ring when a=0, as isshown in FIG. 8a, while there will be an approximately asymptoticincrease when a approaches the size of r.

[0065] The foregoing examples just show planetary gears having threeplanet wheels, one of which is positioned in the characteristic manneraccording to the invention. It is evident that more than three planetwheels, e.g. four, may be used, of which one or two are arranged in thecharacteristic manner according to the invention, while the others arearranged without play or substantially without play on the respectivestays.

[0066] It is moreover clear that other embodiments than those describedmay occur within the scope of the following claims. Thus, e.g. the outerring 4 may be provided with holes instead of the pins 5, while thecarrier 1 may correspondingly be provided with pins instead of holes 3.Moreover, other forms of bearings between the stays and the planetwheels than the shown slide or roller bearings may occur.

[0067] The planetary gear according to the invention may be used inconnection with e.g. internal combustion engines, as mentioned,Generally, however, the invention may be applied where a transfer ofmoment having a great gear ratio, e.g. up 13:1, and/or a relativelygreat transfer of moment is required, in particular where the spaceavailable for a gearbox is relatively scarce. Further, the invention maybe applied in cases where the transfer must allow sliding or slippingbetween the two rotary movements, so that these movements are not lockedto each other, and finally the invention may be applied with anincorporated overload protection, as described previously.

[0068] Finally, the invention may be used in connection with specialapplications, an example of which being transfer of moment in vehicleswhere a force-generating unit, such as an electric motor, isincorporated in or at one or preferably several wheels. Here, the momentis to be transferred to the wheel within a desired range of revolutionsand using a mass as small as possible, since this mass will form part ofthe non-suspended weight at the wheel structure. The planetary gearaccording to the invention may thus be used to great advantage here. Theplanetary gear according to the invention may also be used to advantagein connection with other similar structures or energy-transferringsystems.

1. A planetary gear having a central, rotatably mounted sun wheel, atlast three planet wheels arranged around the sun wheel which arerotatable about their own axes, means for resiliently pressing theperipheral surfaces, which are in the form of faces of revolution, ofthe planet wheels into force-transferring engagement with the peripheralouter face of the sun wheel, characterized in that at least one andmaximum two of the planet wheels (12) is/are mounted such that itsaxis/their axes of rotation is/are substantially fixed relatively to theaxis of rotation of the sun wheel (2), while each of the other planetwheels (6) is arranged on a shaft part (7) extending with apredetermined play into a central bore provided in the planet wheelconcerned, such that the radial distance between the centre line of theshaft part (7) and the axis of rotation of the sun wheel (2) is smallerby a predetermined value than the corresponding radial distance betweenthe centre line of the planet wheel (6) concerned and the axis ofrotation of the sun wheel (2) when the planetary gear is in itsoperational state, in which all the planet wheels (6, 12) are inengagement with the sun wheel (2), and each of the shaft parts is inengagement with the inner surface of the corresponding bore.
 2. Aplanetary gear according to claim 1, characterized In that the givendistance is in excess of the normal accuracy of the machining,preferably in excess of about 0.01 mm.
 3. A planetary gear according toclaim 1 or 2, characterized In that the given distance is in a range ofsubstantially 0.1-2.0 per thousand of the radial dimension of theplanetary gear and preferably In a range of substantially 0.5-1.0 perthousand.
 4. A planetary gear according to one or more of claims 1-3,characterized in that the planet wheel or the other planet wheels (6) aswell as the associated stay or stays (7) is/are constructed such thatthe stay or stays (7) is/are formed with an outer diameter which issmaller than the inner diameter of the corresponding hole (6′) in theplanet wheel or wheels (6), the difference in diameters being in excessof the normal machining accuracy, preferably in excess of about 0.01 mm.5. A planetary gear according to one or more of claims 1-3,characterized in that the planet wheel or the other planet wheels (6) aswell as the associated stay or stays (7′) is/are constructed such thateach stay (7′) comprises an eccentric (15) mounted rotatably thereon. 6.A planetary gear according to claim 5, characterized in that the centreof the stay (7′) is positioned closer to the centre of the sun shaftthan the centre of the associated eccentric (15).
 7. A planetary gearaccording to claim 5, characterized in that the centre of the stay (7′)is positioned further away from the centre of the sun shaft than thecentre of the associated eccentric (15).
 8. A planetary gear accordingto one or more of claims 1-7, characterized in that the planetary wheels(6, 12) are constructed substantially identically, and that they arepreferably constructed as bearings, preferably roller bearings whoseinner diameter substantially corresponds to the outer diameter of thestays (7), and whose outer bearing ring serves as a planet wheel.
 9. Useof a planetary gear according to one or more of claims 1-8 fortransferring a moment at high gear ratios, e.g. up to 13:1, and/or at arelatively grat transfer of moment using a relatively small volume, suchas for transferring a moment to compressors for internal combustionengines or for transferring a moment to propulsion wheels in vehicles.